This invention relates to a communication system and to a central station and to a sub-station in such a system. It arose in the design of a trunked mobile telephone system but would also be applicable to other situations such as multi-user computer systems, and satellite communication systems.
In such a communication system some facility needs to be provided to prevent sub-stations currently having messages to transmit from all doing so simultaneously.
An established technique for doing this (known as dynamic framelength slotted ALOHA) uses a base station which transmits synchronising signals defining the beginnings of "Frames" each containing a specified number of time slots, this number being given in a code which forms part of the synchronising signal. A sub-station having a message to transmit notes the number of time slots in a frame (as given by the code) and transmits its message in one of those time slots which it selects according to some predetermined rule, as described in U.S. Pat. No. 4,398,289, the contents of which are incorporated herein by reference. This system has the advantage that the changes of any two sub-stations transmitting simltaneously are reduced by an amount which depends on the frame length. This frame length can be varied by the main station according to prevailing conditions: a short frame length being preferred in conditions of light traffic to reduce the time that a sub-station needs to wait before transmitting its message; and a long frame length being preferred in conditions of heavy traffic to reduce the chances of two or more sub-stations transmitting in the same time slot.
This invention arose from the realisation that the frequency of the synchronising signals can be used by the sub-station to define the number of slots in a frame and that it is therefore unnecessary for the main station to transmit or for the sub-stations to receive the aforementioned code.